The present disclosure relates to a touch sensing circuit and a semiconductor device including the same, especially those which can be suitably used for touch sensing arranged to sense the approach of a conductor to a sensor capacitance by measuring a response signal obtained from the sensor capacitance according to a detection signal applied thereto.
Touch sensing methods of a capacitance type are classified into a self-capacitance type method and a mutual capacitance type method. In any method, the presence or absence of a touch is detected by applying a predetermined detection signal to a sensor capacitance, charging or discharging a sensor capacitance and a capacitance component decreased or increased by touching, and receiving and measuring a response signal resulting therefrom.
A mutual capacitance type touch sensing circuit is disclosed by the Japanese Unexamined Patent Application Publication No. JP-A-2014-106864, in which a detection signal is assumed to be a rectangular wave, and a response signal corresponding to a rising edge thereof, and a response signal corresponding to the falling edge are integrated after mutual polarity inversion, whereby the sensitivity of detection is increased.
The U.S. Patent Application Publication No. 2010/0295715 discloses a delta-sigma (ΔΣ) modulation circuit including, in its input part, a plurality of capacitances with a plurality of switches, smoothing an input signal. In the modulation circuit, the feedback control of the switches is performed so as to make a DC component of the input signal zero, thereby reducing an error owing to the clock jitter.
In JP-A-2014-106864, there is enough time in the timing of inverting the polarity of a response signal in the case of using a rectangular wave as the detection signal; and in the case of using, as the detection signal, a sine wave, a triangular wave or the like, there is not enough time in the timing of inverting the polarity of a response signal. In case that there is not enough time in the timing of inverting the polarity of a response signal, a response signal which is originally on the positive side can be integrated as a response signal on the negative side or vice versa. As a result, the detection or sensing accuracy cannot be maintained. Specifically, there is the problem that a phase noise component superimposed on a response signal worsens the sensing accuracy.
To solve the problem, a technique described in U.S. Patent Application Publication No. 2010/0295715 may be combined. Specifically, the timing of inverting the polarity of a response signal can be kept at an appropriate timing by providing a plurality of capacitances with respective switches in the response signal input part, and performing the feedback control of the switches so as to make a DC component of the response signal zero.
However, the solving means like this has the problem that the necessity for providing a plurality of capacitances and switches leads to the increases in circuit scale. Especially, for a touch sensing circuit, many sensor capacitances are mounted thereon because of the need for increasing the sensing accuracy of touch coordinates and accordingly, there is a tendency to mount many touch sensing circuits. So, the increase in circuit scale becomes a serious problem.